Dual element cartridge fuse for small current intensities



Sept. 19, i967 F J. KOZACKA DUAL ELEMENT CARTRIDGE FUSE FOR SMALL CURRENT INTENSITIES Filed March 24, 1967 INVENTOR:

FREDERECK J. KOZACKA BY Mm Wm ATTORNEY United States Patent 3,342,964 DUAL ELEMENT CARTRIDGE FUSE FOR SMALL CURRENT INTENSITIES Frederick J. Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed Mar. 24, 1967, Ser. No. 625,722 4 Claims. (Cl. 200-123) ABSTRACT OF THE DISCLOSURE Field of invention The invention relates to an adaptation for small current intensities of the structure disclosed and claimed in U.S. Patent 2,321,711 to Elmer H. Taylor, issued June 15, 1943, for Fusible Electric Protective Device.

. The above structure includes a spring housing, a spring inside said housing, a plunger biased by said spring and a solder joint normally maintaining said plunger in position against the bias of said spring. Upon softening of the solder joint by relatively small overloads of excessive duration the solder joint yields to the action of the spring and the plunger is moved under theaction of the spring into the spring housing, thus establishing a circuit interrupting breakxThe sprin'g'housing is arranged at the center region of a casingfof insulating material filled with a pulverulent arc-quenching filler. A first fuse link conductively connects the plunger with one of the terminal l'ements'of the fuse and a second fuse link-conductively 3,342,964- Patented Sept. 19, 1967 The arrangement of the time-delay interrupting means at the aforementioned point of the fuse structure, i.e. close to one of the terminals thereof, tends to result in an intense heat flow away from the time-delay interrupting means which, in turn, tends to keep the temperature of the solder joint between the plunger and the fusible ele ment at temperatures which are too low after protracted small overloads to result in the required time-current characteristic. This tendency is effectively counteracted by the provision of a thermal barrier, or equivalent thermal insulating means, between the spring housing and the terminal element .of the fuse immediately adjacent the spring housing.

Brief description of the drawings Description of preferred embodiment In the structure shown in the drawings numeral 1 has been applied to indicate a casing of insulating material, e.g. fiber. Casing 1 is closedv on both ends there-of by terminal elements 2, preferably in the form of caps or ferrules. Casing 1 is filled with a pulverulent arc-quenching filler 3 as, for instance, quartz sand. The fuse further includes composite fusible means inside of easing 1 subconnects the spring housing with the other terminal element of'the fuse. If the spring housing is spaced from the internal surface of the casing of the fuse there is a tendency that the first fuse link, and the second fuse link, are subjected to considerable mechanical stresses, resulting from the aggregate weight of the plungerfthe spring housing and the spring. These stresses are within tolerable limits as long'as the fuse links are of relatively large cross-section, which is always the case when the fuse is designed to carry relatively large currents. In fuses designed for small currentintensities the elongated fusible elements, or fuse links, are relatively thin and as a result not capable of withstanding the stresses imposed by the aggregate weight of the plunger, the spring housing and the spring. On account of this fact the structure disclosed in the above referred-to patent has, heretofore, been confined to fuses; having relatively large current ratings. Other, less desirable designs were used for fuses intended for fractions'of anamp.,'up to afew amps, and lacking advantagesinherent'in the structure disclosed and claimed in the above patent.

Summary of invention v 7 According to this invention the plunger, the spring housing and the spring are arranged immediately adjacent, or in close proximity to, one end of the casing of p mersed in filler 3. Reference numeral 4 has been applied to indicate an elongated fusible element, or fuse link, having a relatively small cross-section and consequently a relative- 1y small current-carrying capacity. The end 4a of fusible element 4 is spaced. from the upper cap or ferrule 2, and the end 4b of fusible element 4 is situated immediately adjacent the lower cap or ferrule 2, projects through an opening in the lower cap or ferrule 2 and. is conductively connected with the lower cap or ferrule 2 by a solder joint 5. Reference numeral 6 has been applied to indicate a plunger having a flange 7 under the action of a helical spring 8. Spring 8 is arranged inside of cylindrical spring housing 9 of metal. Spring housing 9 includes a radially 1 outwardly extending flange 9a forming an integral part thereof. Spring housing 9 is covered with a layer'10 ,of heat resistant insulating material, and supports a helical heater windingll. A first solder joint 12 establishes a conductive connection between the lower end of heater winding 11, and spring housing 8, and further establishes a mechanical tie between plunger 6 and spring housing 9, normally precluding a movement of plunger 6 under the action of spring 8. The axially inner end of plunger 6 is slotted at 6a, as shown in FIG. 2, and the upper end 4a of fusible element or fuse link 4 is inserted into slot 6a and connected with plunger 6 by means of a second solder joint 13. The latter establishes a conductive connection between the upper end 4a of fusible element 4 and plunger 6 and a mechanical tie between the upper end of fusible element 4 and plunger 6. Flange 9a is arranged in abutting relation to a washer 14 of fiber, or the like thermal insulating material, provided with a central aperture 14a for the passage of spring housing 9'. Washer 14 is further provided with an off-center aperture 14b of. smaller size than aperture 14a for the passage of the. upper, end. of h'e'at'er winding" 11. The upper end surface of 'fla'ng'e9iz is engaged by a washer 15 of insulating material, e.g. fiber, arranged parallel to, and spaced from, washer 14. Washer 15 is provided with an off-center aperture 15a arranged in registry with the aperture in washer 14. The upper end of winding 11 extends also through aperture 15a in 3 washer 15, and through an opening in the upper cap or ferrule 2 to the outer surface of the latter. There it is conductively connected to cap or ferrule 2 by means of solder joint 16. The end surface of the upper cap or ferrule 2 is provided with a recess forming a well for solder joint 16 and also forming a spacer for spacing washer 15 from the end surface of the upper cap or ferrule 2. Thus a first thermal insulating layer 17 is established between the end surface of the upper ferrule 2 and washer 15. ,A second thermal insulating layer is established between washers 14 .and 15. The first mentioned thermal insulating layer 17 minimizes axial heat flow from spring housing 9 and the flange 9a thereof, and the second mentioned insulating layer 18 minimizes radial heat flow from spring housing 9 and its flange 9a to cap 2 adjacent thereto.

The fuse shown and described establishes normally the following current path: Upper terminal cap 2, solder joint 16, heater coil '11, solder joint 12, plunger 6, solder joint 13, fusible element 4, solder joint 5, lower terminal cap 2.

On occurrence of short-circuit currents fusible element 4 fuses and vaporizes, thus establishing a circuit inter rupting break.

n occurrence of small protracted overload currents heater element 11 heats spring housing 9 and plunger 6 to such an extent that both solder joints 12 and 13 soften. Hence plunger 6 is free to move under the action of spring 8 axially outwardly into housing 9, thus establishing a break between the upper end 4a of fusible element 4 and plunger 6.

The wattage dissipated by heater winding 11 is relatively small since the structure 6, 9 is thermally well insulated. The weight of parts 6, 8, 9 is entirely supported by washers 14 and 15, and thus does not subject the fragile fuse link, or fusible element, 4 to undue stresses and strains. Fusible element 6 may take the form of a thin wire where a particularly small current-carrying capacity, or current rating, is desired. The large solder joint 12 interconnecting plunger 6 and spring housing 9 is subject to most of the force exerted by spring 8, and thus relieves the smaller solder joint 13 from excessive stresses and strains.

It will be understood that though but one embodiment of the invention has been illustrated and described in detail, the invention is not limited thereto. It will also be understood that the structure illustrated may be modified without departing from the spirit and scope of the invention as set forth in the accompanying claims.

I claim as my invention:

1. An electric cartridge fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a pulverulent arc-quenching filler inside said cas- ((1) composite fusible means inside said casing submersed in said arc-quenching filler conductively interconnecting said pair of terminal elements, said composite fusible means including an elongated element of a relatively small current-carrying capacity having one end immediately adjacent one of said pair of terminal elements and another end spaced from the other of said pair of terminal elements, said fusible means further including a solder joint conductively interconnecting said another end of said elongated fusible element and a spring biased pl ng (e) a tubular metal housing inside said casing accommodating one end of said plunger and a spring biasing said plunger, said metal housing being arranged in close proximity to said other of said pair of terminal elements;

(f) a heater coil arranged on the outside of said metal housing and supported by said metal housing, one end of said heater coil being conductively connected to said plunger and the other end of said heater coil being conductively connected to said other of said pair of terminal elements; and

(g) means of thermal insulating material supporting said metal housing and thermally insulating said metal housing from said other of said pair of terminals.

2. An electric cartridge fuse as specified in claim 1 wherein said means supporting said metal housing establish a radial air gap and an axial air gap between said metal housing and said other of said pair of terminal elements.

3. An electric cartridge fuse as specified in claim 1 including (a) a radially extending flange on said metal housing integral therewith arranged on the side thereof adjacent said other of said pair of terminal elements; and

(b) a pair of washers of thermal insulating material arranged coaxially inside said casing and in spaced parallel relation, said flange on said housing being clamped between said pair of washers and positioned by said pair of washers, one of said pair of washers defining an aperture for the passage of a portion of said housing projecting from said pair of washers axially inwardly into said casing.

4. An electric cartridge fuse as specified in claim 1 including (a) a pair of terminal elements in the form of a pair of caps mounted on the outer surface of said casing at the ends thereof;

(b) a first washer of thermal insulating material inside of one of said pair of caps in abutting relation with one rim of said casing, said first washer defining a relatively large central aperture for the passage of one end of said housing and a relatively small oif center aperture for the passage of one end of said heater coil;

(c) a second washer of thermal insulating material inside said one of said pair of caps arranged parallel to and in spaced relation from said first washer and in spaced relation from the end surface of said one of said pair of caps, said second washer defining a relatively small off-center aperture for the passage of said one end of said heater coil and being aligned with said off-center aperture defined by said first washer; and

(d) a circular flange on the axially outer end of said metal housing arranged inside of the space bounded by said first washer and said second washer and positioned by said first washer and by said second Washer.

References Cited UNITED STATES PATENTS BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

1. AN ELECTRIC CARTRIDGE FUSE COMPRISING IN COMBINATION: (A) A TUBULAR CASING OF INSULATING MATERIAL; (B) A PAIR OF TERMINAL ELEMENTS CLOSING THE ENDS OF SAID CASING; (C) A PULVERULENT ARC-QUENCHING FILLER INSIDE SAID CASING; (D) COMPOSITE FUSIBLE MEANS INSIDE SAID CASING SUBMERSED IN SAID ARC-QUENCHING FILLER CONDUCTIVELY INTERCONNECTING SAID PAIR OF TERMINAL ELEMENTS, SAID COMPOSITE FUSIBLE MEANS INCLUDING AN ELONGATED ELEMENT OF A RELATIVELY SMALL CURRENT-CARRYING CAPACITY HAVING ONE END IMMEDIATELY ADJACENT ONE OF SAID PAIR OF TERMINAL ELEMENTS AND ANOTHER END SPACED FROM THE OTHER OF SAID PAIR OF TERMINAL ELEMENTS, SAID FUSIBLE MEANS FURTHER INCLUDING A SOLDER JOINT CONDUCTIVELY INTERCONNECTING SAID ANOTHER END OF SAID ELONGATED FUSIBLE ELEMENT AND A SPRING BIASED PLUNGER; (E) A TUBULAR METAL HOUSING INSIDE SAID CASING ACCOMMODATING ONE END OF SAID PLUNGER AND A SPRING BIASING SAID PLUNGER, SAID METAL HOUSING BEING ARRANGED IN CLOSE PROXIMITY TO SAID OTHER OF SAID PAIR OF TERMINAL ELEMENTS; (F) A HEATER COIL ARRANGED ON THE OUTSIDE OF SAID METAL HOUSING AND SUPPORTED BY SAID METAL HOUSING, ONE END OF SAID HEATER COIL BEING CONDUCTIVELY CONNECTED TO SAID PLUNGER AND THE OTHER END OF SAID HEATER COIL BEING CONDUCTIVELY CONNECTED TO SAID OTHER OF SAID PAIR OF TERMINAL ELEMENTS; AND (G) MEANS FOR THERMAL INSULATING MATERIAL SUPPORTING SAID METAL HOUSING AND THERMALLY INSULATING SAID METAL HOUSING FROM SAID OTHER OF SAID PAIR OF TERMINALS. 